Artist's medium beat an ocean smear (3-5)

I believe the answer is:

oil-slick

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'an ocean smear' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'artist's medium beat' is the wordplay.
'artist's medium' becomes 'oils' (I've seen this in another clue).
'beat' becomes 'lick' (synonyms).
'oils'+'lick'='OIL-SLICK'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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